The present invention relates to acoustic wave devices, such as SAW devices, which are useful e.g. as filters and oscillators, and is more particularly directed to a technique of packaging these devices which is both economical and rugged. The technique is also applicable to other electronic devices.
The invention is more specifically with a technique for creating acoustic devices and packaging the same at the wafer level before singulation or dicing. This permits the devices to be packaged and tested before they are singulated into individual units, thus avoiding many of the reasons for high costs in fabrication.
SAW (Surface Acoustic Wave) devices, STW (Surface Transverse Wave) devices, or similar acoustic wave devices are commonly used in various applications, such as in spectral filtering of electronic signals. The most common type of these are the SAW devices. In the SAW device, any of various techniques are used to launch a Rayleigh wave, and subsequently receive it after it has traveled along a predetermined path along a prepared region of a substrate material, such as quartz crystal, lithium niobate, or lithium tantalate. Usually, there are metal electrodes connected to some metallized termination, using wire bonds. The surface along which the acoustic wave travels is quite sensitive, and this sensitivity becomes extreme as the frequency of operation increases. Traditionally, the package in which the device is encapsulated is hermetically sealed.
Currently, SAW devices are placed in pre-made packages of ceramic or metal. This means that the devices themselves are singulated prior to that time and have to be inserted into their individual packages. A spot of mounting adhesive on the underside holds the SAW in place. Typically, a head space of air or nitrogen is provided above the upper surface.
These packages are typically pre-fabricated by an outside supplier. Because each of the dies has to be installed individually into its own package, the step of packaging can be quite costly. Typically, the cost of the finished SAW device is limited by the cost of the package. Also, the degree to which the size of the device can be reduced is also limited by the requirement for head space. It would be advantageous to employ surface-mounted acoustic wave devices in many circuits, both for purposes of miniaturization and to eliminate sources for unwanted signal reflections. However, effective surface mounting has been difficult to achieve with traditional packaging techniques.
One technique for creating and packaging surface acoustic wave devices is described in Kong et al. U.S. Pat. No. 5,448,014. This patent relates to a hermetically sealed flip-chip device, in which dies are formed on a first wafer, and conductive vias are formed through a second wafer. The solder grids are formed on the first wafer. The two wafers are faced against each other and the solder grids are reflowed to join the two wafers. The solder grids serve to connect the conductive terminal pads of the dies to the vias, and also serve to seal the dies between the wafers. The dies are tested at the wafer level, and then saw cut into individual acoustic devices. While the Kong et al. technique does create some efficiencies in the manufacture of these acoustic devices, it nevertheless creates other problems. First, the solder grids used at the periphery of the devices for hermetic sealing create a conductive ring, and can affect the electrical and electromagnetic properties of the die that is encapsulated there. Further, laying down of a solder grid on top of other supporting metallic layers can be complex and expensive. Also, this technique requires a rather precise and even application of heat at a high enough temperature to reflow the solder grid without damaging the dies. Further, because the solder grid both effects electrical connections and creates a hermetic seal, the grid pattern has to be somewhat complex in order to isolate the peripheral solder sealing ring from the solder terminals.
A number of anisotropic conductive adhesive compositions are available for use in connection with electrical or electronic devices, but these have not been employed in the packaging of acoustic wave devices. Basically, an anisotropic conductive material comprises a thermoplastic carrier or matrix, and a large number of electrically conductive particles dispersed in the carrier or matrix. The material can be in a variety of forms, depending on the preference of the manufacturer, such as a film that can be applied as a tape, or a paste or ink that can be applied by thick-film techniques, such as screening or stenciling. The material is sandwiched between an upper substrate and a lower substrate, where there are conductors on the two substrates in registry, i.e., in vertical alignment with one another. Then the sandwiched workpiece is subjected to pressure and heat, and the matrix softens and then sets to join the two substrates. The metallic particles become pressed into the upper and lower conductors, and form a vertical conductive pathway between them, i.e., in the Z direction. On the other hand, the conductive particles remain out of contact in the plane parallel to the two substrates, and do not form conductive paths in the X or Y directions.
The application of anisotropic conductive adhesive materials to packaged acoustic devices could in theory serve both as the sealing means and as the conductive path between the die and the outer electrode conductors. However, the organic matrix or carrier is somewhat permeable to gasses and does not create a completely hermetic seal. In addition, there is some outgassing over time from the organic material itself which may also compromise the surface of the die.